/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;

import java.util.Collections;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * Implementation of an {@link Optional} not containing a reference.
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
final class Absent<T> extends Optional<T>
{
    static final Absent<Object> INSTANCE = new Absent<>();

    @SuppressWarnings("unchecked") // implementation is "fully variant"
    static <T> Optional<T> withType()
    {
        return (Optional<T>) INSTANCE;
    }

    private Absent()
    {
    }

    @Override
    public boolean isPresent()
    {
        return false;
    }

    @Override
    public T get()
    {
        throw new IllegalStateException("Optional.get() cannot be called on an absent value");
    }

    @Override
    public T or(T defaultValue)
    {
        return checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)");
    }

    @SuppressWarnings("unchecked") // safe covariant cast
    @Override
    public Optional<T> or(Optional<? extends T> secondChoice)
    {
        return (Optional<T>) checkNotNull(secondChoice);
    }

    @Override
    public T or(Supplier<? extends T> supplier)
    {
        return checkNotNull(
                supplier.get(), "use Optional.orNull() instead of a Supplier that returns null");
    }

    @Override
    @CheckForNull
    public T orNull()
    {
        return null;
    }

    @Override
    public Set<T> asSet()
    {
        return Collections.emptySet();
    }

    @Override
    public <V> Optional<V> transform(Function<? super T, V> function)
    {
        checkNotNull(function);
        return Optional.absent();
    }

    @Override
    public boolean equals(@CheckForNull Object object)
    {
        return object == this;
    }

    @Override
    public int hashCode()
    {
        return 0x79a31aac;
    }

    @Override
    public String toString()
    {
        return "Optional.absent()";
    }

    private Object readResolve()
    {
        return INSTANCE;
    }

    private static final long serialVersionUID = 0;
}
